The present invention relates to a method for the manufacture of resist structures of the positive resist type by means of short-wave UV rays.
The manufacture of positive resist structures by means of short-wave UV radiation (deep UV rays) is known, the range of between about 180 and 260 nm being used with preference. The resist structures or masks produced, which preferably have a thickness of about 1 .mu.m, are used especially in the fabrication of integrated circuits.
For manufacturing resist structures of the type mentioned, a number of resist materials is already being used. One of these materials is polymethylmethacrylate (PMMA). This resist material is distinguished by high contrast and is sufficiently thermostable for use in such thermal processes in semiconductor technology in which temperatures above 135.degree. C. are not exceeded. If commercially available short-wave UV lamps such as mercury low pressure lamps, xenon-mercury high pressure lamps or deuterium lamps are used, however, PMMA is too insensitive, i.e., excessively long exposure times are required for forming resist patterns. In the manufacture of integrated circuits by irradiation, it is extremely important to hold as low as possible the energy dose and the exposure time required for sufficient breaking down of polymer material so that the process can be carried out practically as well as economically within justifiable limits.
Polybutene sulfone (PBS), while about 100-times more sensitive to UV rays at 184 nm than PMMA, is not suitable for the thermal processes being practiced in semiconductor technology, such as "lift-off" processes, dry etching processes and ion implantation, because of the low dimensional heat stability and the thermal instability (depolymerization at temperatures of about 70.degree. C.). A further disadvantage of the positive resist PBS is the excessively high absorptivity for short-wave UV rays, which leads to very limited resolution, such that the resist masks of 1 .mu.m thickness, frequently required for structuring processes, can no longer be realized for transferring very small structures onto semiconductor substrates.
Other resist materials such as poly-2,2,3,4,4,4-hexafluorobutylmethacrylate and polymethylisopropenyl ketone, while about 5 to 6 times more sensitive than PMMA and therefore suitable for structuring resist films 1 .mu.m thick, have a dimensional heat stability (glass or glass transition temperature T.sub.g .about.50.degree. C. and 62.degree. C., respectively) which is likewise so low, however, that they are not suited for thermal processes such as occur in semiconductor technology.
It already has been attempted to use a commercially available photographic varnish of the novolaks and diazoquinones type as the deep-UV resist because of the excellent etching resistance and thermal stability. A particular disadvantage of such a material, however, is an extremely high UV absorption in the application range of interest below 260 nm, so that only varnish films less than 0.5 .mu.m thick can be structured with relatively steep flanks. Films of such thickness, however, have only very limited etching resistance. A further disadvantage is the high spectral sensitivity of the varnish above 300 nm, i.e., high resolution can be achieved only through the use of either interference filters which are expensive and heavily reduce the intensity, or appropriately surface-coated reflection mirrors.
It also has been attempted to sensitize PMMA, while retaining its other good resist properties, to thereby increase the sensitivity about 5 times. The sensitizers are liquid benzoic acid derivatives which absorb in the typical emission range of, for example, a xenon-mercury short-arc lamp, and pass the energy to the PMMA, and thereby cause more chain breaks. However, if a polymethylmethacrylate sensitized in this manner is used, adverse effects on the resist properties can result with consequent limitations of the applications. In particular, if multiply-structured substrates with a height profile are varnished by means of the centrifuging technique, the structure corners receive less varnish than the remainder of the substrate surface. The danger exists, therefore, that in the centrifuging process and in the so-called "prebake", the liquid sensitizer evaporates faster at the less varnished stages and, therefore, different sensitivity regions are formed on the substrate, which can have a substantial adverse effect on the resolution. A similar disadvantageous effect on the quality of the structure can be caused by the liquid or highly volatile additives during the so-called "post-bake". In the case of thicker varnish layers, there is furthermore the danger that so-called "pinholes" are formed during the evaporation of the sensitizer.
In the efficient manufacture of positive resist structures, preferably about 1 .mu.m thick, by means of short-wave UV rays, the following requirements should be met:
a low UV dose should be required for structuring; PA1 the structure should have steep resist flanks; PA1 the resist structure should be chemically resistant to wet etching processes used in semiconductor technology, and thermally resistant in dry etching and "lift off" processes; PA1 high resolution should be ensured by utilizing the UV spectral range below 260 nm and by the particular spectral absorptivity of the resist material; PA1 through making the resist transparent above 260 nm, the expensive and intensity-reducing interference filters or appropriately surface-coated reflection mirrors otherwise required for eliminating the longer-wave UV lamp emission region should be dispensed with. PA1 (a) 1 to 70 mol % alkylmethacrylate with an alkyl radical having 1 to 4 C atoms; and PA1 (b) 99 to 30 mol % of an ethylenically unsaturated monomer with chlorine and/or cyan substituents. PA1 (a) First, a film of an (uncrosslinked) polymer material with the above-described composition is exposed with a predetermined pattern to short-wave UV rays in the range between about 180 and 260 nm; PA1 (b) The exposure to short-wave UV light is continued until the polymer material is broken down (in the exposed zones) into low-molecular products or into polymers having better dissolution properties; and PA1 (c) The breakdown products in the exposed regions then are removed. PA1 The radiation with short-wave UV light takes place here under oxygen exclusion, preferably in an inert gas or in a vacuum.
As pointed out above, these requirements in their totality heretofore have been met only unsatisfactorily, since only part of the mentioned requirements can be achieved at any one time with the resist materials used to date.